Polyolefins, such as polyethylene, may be prepared by particle form polymerization, also referred to as slurry polymerization. In this technique, a monomer feed is polymerized in a reaction zone in the presence of a catalyst and a diluent to produce a polymerization effluent containing a slurry of polymer solids in the diluent. In order to recover the polymer from the slurry, the polymerization effluent is withdrawn from the reactor and the polymer solids are separated from the liquid diluent. Typical polymer recovery and separation systems include subjecting the polymerization effluent to a reduction in pressure so that the liquid diluent vaporizes leaving concentrated polymer solids. Often the reduction in pressure occurs in a first high pressure flash tank and a further reduction may occur in a second lower pressure flash tank. The vaporized diluent exits at a top portion of a flash tank, while the polymer remains solid and is recovered through a bottom portion of the flash tank. The vaporized diluent can then be recycled back to the reaction zone.
In addition to polymer solids and diluent, the polymerization effluent discharged from the reactor can also contain active catalyst and unreacted monomer. The high pressure and temperature conditions in the polymer recovery and separation system can be conducive to further polymerization. Further polymerization may then lead to polymer growth and fouling within the polymer recovery and separation system. The fouling may then lead to plugging of the polymer recovery and separation system in as little as a few hours and as long as several months, which may ultimately lead to stoppages in polyolefin production while the system is cleaned. Therefore, there is a need for a way to limit and/or prevent continued polymerization within the separation system, so as to reduce or eliminate fouling and plugging.
“Kill” systems and catalyst deactivating agents for olefin polymerization catalyst systems are known in the art. For example, U.S. Pat. No. 7,381,777 discloses methods for controlling fouling in the reaction zone of slurry-type olefin polymerization reactors by measuring a process operating parameter and by responding to said process operating parameter, e.g., by introducing a gaseous poison for the polymerization catalyst in response to a comparison between said measured parameter and a fouling parameter limit for said measured parameter. Such “kill” systems typically employ large amounts of catalyst poison and are intended to completely halt all polymerization within the reactor.
U.S. Pat. No. 4,211,863 discloses a method for deactivating catalyst residue in a polymer slurry. The catalyst residues of a transition metal compound and an organometallic compound contained in a polymer are deactivated by contacting the polymer with anhydrous carbon dioxide in a transfer line connecting the reaction zone with a zone of sufficiently lower pressure such that flashing of diluent occurs in the transfer line to assure mixing of the carbon dioxide and the catalyst residue. The flashing occurs in a low pressure flash chamber that operates at a pressure ranging from atmospheric pressure up to about 3 psig. Large amounts of carbon dioxide are needed to sufficiently deactivate the catalyst residue to inhibit the corrosivity, odor, and coloring of the final polymer product associated with the residual amounts of transition metal and organometallic catalysts.
Thus, there still remains a need for a way to control fouling outside of the reactor, particularly in the polymer recovery and separation system. The method should reduce or inhibit fouling and plugging in the polymer recovery and separation system, but should not diminish reactor production rates.